The invention relates to an thermoplastic molding and more particularly to an injection molding machine
An injection molding machine is disclosed. The machine has an injection unit and nozzle and the injection unit includes a tubular cylinder and a screw that is capable of back and forth movements along the tubular cylinder. Means are provided to stop the movement of the screw at its end/stop position at a distance from the nozzle so as to form a gap between the screw head and nozzle. The gap has a volume of more than       π    4    ·      D    3  
and up to the volume of a shot wherein D is the dimension of a single screw stroke. The use of the machine yields molded articles that are virtually streak free.
Injection molding machines are known. Patent application DE 195 14 070 A1, disclosed a machine having an injection unit and a casting mold, in which the injection unit comprises a screw movable in the tubular cylinder via guide rods, which screw may likewise be controlled with respect to its start and end/stop position in the tubular cylinder. In addition, controlled vibration in the area of the end/stop position is provided. Nothing is disclosed in this document about the residual volume in the area of the end/stop position.
Small amounts of gases in the plastics melt, which are caused by, for example, air during feeding of the plastics pellets, by chemical or physical reactions during melting or by moist material, may be entrained into the space in front of the screw and cause silvery surface streaks on the molding during injection.
In the middle of the flow channel, the gas bubbles reach the melt front of the plastics. The pressure decreases progressively along the direction of the melt front and the gas bubbles become larger. At the melt front itself, normal pressure prevails initially, and the bubbles burst there and are rolled out and stretched on the cold mold wall. (Depending on the color of the plastics), this leads to silvery, U-shaped streaks open towards the flow direction.
Measures taken to prevent streaks Include predrying of the plastics in accordance with which, prior to processing, the plastics material is dried for several hours at an elevated temperature (80xc2x0 C.-120xc2x0 C.) in an additional unit.
Predrying is disadvantageous because additional units are required that increase expenditures on purchase, maintenance and total energy consumption of the injection molding machine. Inadequately dried material causes defects at the molding surface of the above-described type.
Also known is the use of a vented plasticizing unit in which case the gases entrained in the plastics material exit through a vent in the cylinder. The gas bubbles are compressed to a considerable degree in a compression zone, the pressure in the bubbles then amounts to several hundred bar. In the area of the vent, standard atmosphere prevails and the pressure difference leads to bursting of the gas bubbles and the gas escapes through the vent.
Disadvantages of this procedure include difficult process control, possibility of plastics exit through the vent, separate expenditure on a special-purpose plasticizing unit and on additional equipment, together with reduced plasticization performance.
The object of the invention is to provide a device and method that avoid the disadvantages of the present design and known methods and minimize streaking.
In accordance with the invention an injection molding machine having an injection unit and a casting mold. The injection unit comprise a tubular cylinder and a screw that is capable of moving in the tubular cylinder in a known manner via guide means, and where the movement of the screw is controlled with respect to its start and end/stop position in the tubular cylinder. In accordance with the invention, the end/stop position of the screw leaves a residual volume of melt, in the tubular cylinder of more than       π    4    ·      D    3  
and up to at most a melt volume comparable to the shot volume, wherein the cross section D of the screw is the dimension of a single screw stroke.
The invention further provides a method of operating an injection molding machine for processing thermoplastics melts, having an injection unit and a casting mold, in which the injection unit comprises a screw movable in the tubular cylinder via guide means, which screw may be controlled with respect to its start and end/stop position in the tubular cylinder, characterized in that the screw is stopped at an end/stop position on ejection of the plastics melt, in that there remains in the tubular cylinder a residual volume, filled with melt, of more than       π    4    ·      D    3  
up to at most a melt volume comparable to the shot volume, wherein the cross section D of the screw is the dimension of a single screw stroke.
The invention relates in particular to a nozzle adapter which may be fitted in front of the cylinder head of the tubular cylinder and similarly prevents streaking on the surface of the molded article.
The adapter preferably has a volume of more than       π    4    ·      D    3  
up to at most a volume comparable to the shot volume.
The surprising effect of the device and method according to the invention may in principle be explained below as follows: the plastics melt is exposed to a high pressure during the injection process. The gas originating from the pellets and present in the melt then becomes dissolved in the melt. During the next injection process, this melt is injected into the mold (e.g. from the adapter). The gas remains in solution and does not reach the melt front as gas bubbles, the result being that no streaking occurs as is the case with the known methods. Even if the molding volume is greater than the volume of the adapter, no streaking occurs. This may possibly be explained by the pressure increase along the flow path. As mold filling progresses, the pressure in the space in front of the screw, and thus the melt pressure, increases. The higher the pressure, the greater the gas dissolution capacity. Since the melt from the adapter is injected into the mold first, the gas which has been entrained during the new metering process may dissolve before the melt reaches the mold.
The invention may in particular be realized with one of the following structural measures:
The cylinder head of an injection molding machine is provided with an enlarged space in front of the melt.
Between nozzle and cylinder head there is fitted an adapter component for producing an enlarged space in the front of the melt.
The machine nozzle and/or the hot runner of the injection mold is optionally additionally enlarged in order to produce the desired residual melt volume.
The tubular cylinder is in particular constructed to be correspondingly longer than the screw.
The screw stroke may be defined with the aid of a detachable mechanical limit stop. For cleaning purposes, this limit stop may be removed for short periods and refitted after cleaning of the buffer space.
The invention provides the following advantages over the known methods:
Use of a nozzle adapter results in higher process reliability. The volume from the nozzle adapter cannot be injected additionally into the mold, since the screw moves forward only up to a predetermined distance from the cylinder head. Operating errors are thereby avoided. By means of an interposed nozzle adapter, conventional injection-molding machines may be cost-effectively retrofitted without major effort.
Application of the method according to the invention optionally saves predrying of the thermoplastics material, which is processed on the injection molding machine.
Since all melt-processable materials tend to form streaks, the method may be used for all known thermoplastics.
The following thermoplastics may preferably be used: PC, ABS, PBT, SAN, PA, TPU, PET, PS, POM, polyolefins, in particular PP, together with mixtures thereof and mixtures with other plastics, e.g. mixtures of PC and ABS.
In order to reduce surface streaking, a residual buffer of melt corresponding to a volume of more than 1D screw stroke (cross-sectional area of the screwxc2x7length) may in principle be established via machine adjustment.
The usable shot volume in injection molding machines conventionally amounts to approximately 1 to 3 times the screw diameter (1D to 3D) multiplied by the cross section of the tubular cylinder.
Above a volume corresponding to a stroke, of more than 3D multiplied by the stroke length increased streaking on the moldings is observed in conventional machines.
The usable shot volume may be increased in streak-free manner when the device according to the invention is used up to a volume corresponding to at least 4D stroke of the screw.
This results in a considerable economic advantage, since smaller injection molding machines may be used to produce larger moldings.
Establishment of a residual buffer of melt by machine adjustment of the screw is not preferable, because, in the case of operating errors, the large residual buffer of melt may also be injected into the (casting mold) and machine and mold may thus be damaged.
The invention is explained in more detail below with reference to the Figures on the basis of an Example, which, however, does not limit the invention in any way.